Duct Opening Size vs Vent Cover Size
Duct opening size is the hole behind the vent, and vent cover size is the finished face that sits over it. Supply registers, return air grilles, and floor registers are sold by the duct opening size. So the answer to what size vent cover do I need is the size of the hole, not the old faceplate.
Measuring the face instead is the mistake behind many wrong-size orders and returns. The face typically runs 1-3/4" larger than the opening on steel supply registers and return air grilles, 2-5/8" larger on steel return air filter grilles, and 1-3/8" larger on toe kick registers. A 12" x 6" register, for example, has a 13-3/4" x 7-3/4" face.
This guide covers how to measure the duct opening, how much larger the face runs on each vent type, and how to convert a face measurement you already took into an order size. It also lists the vents ordered by neck size or ceiling grid instead, how vent cover sizes read on a floor register, and where the standard vent and register sizes fit in. The first section gives the short answer on how to measure a vent cover.
Duct Opening Size vs Vent Cover Size
Duct opening size is the width and height of the hole in the wall, ceiling, or floor, while vent cover size is the outside edge of the finished face. Remove the old cover first, then follow the steps for how to measure the duct opening on each vent type.
Vents are sold by the opening because the opening is the fixed part of the job. The duct boot, the drywall cut, and the floor cut stay where they are, while face widths change from one model to the next. A 12" x 6" steel supply register fits a 12" x 6" opening, and its face measures 13-3/4" x 7-3/4". That extra 1-3/4" is not the same on every vent type.
How Much Larger the Face Runs on Each Vent Type
The face runs 1-1/4" to 2-5/8" larger than the duct opening on most vent types, and the gap is usually consistent within a product line. The common openings for each type sit in the standard vent and register sizes chart, and the gaps below compare the listed opening with the published overall face dimensions of standard off-the-shelf models.
The table below lists how much larger the face runs overall, with one example size for each vent type.
| Vent type | Face vs. duct opening | Example: opening and face |
|---|---|---|
| Steel or aluminum supply register | 1-3/4" larger | 12" x 6" opening, 13-3/4" x 7-3/4" face |
| Steel return air grille | 1-3/4" larger | 10" x 10" opening, 11-3/4" x 11-3/4" face |
| Steel return air floor grille | 1-3/4" larger | 8" x 12" opening, 9-3/4" x 13-3/4" face |
| Steel baseboard return grille | 1-3/4" larger | 12" x 6" opening, 13-3/4" x 7-3/4" face |
| Aluminum eggcrate return grille | 1-3/4" larger | 8" x 32" opening, 9-3/4" x 33-3/4" face |
| Aluminum square ceiling diffuser | 1-3/4" larger | 8" x 8" opening, 9-3/4" x 9-3/4" face |
| Aluminum linear bar grille | 1-13/16" larger | 8" x 8" opening, 9-13/16" x 9-13/16" face |
| Aluminum linear slot diffuser | 1-7/16" to 1-7/8" larger, by size | 10" x 6" opening, 11-3/4" x 7-13/16" face |
| Steel return air filter grille | 2-5/8" larger | 16" x 20" opening, 18-5/8" x 22-5/8" face |
| Aluminum eggcrate return filter grille | 2-1/4" larger | 20" x 24" opening, 22-1/4" x 26-1/4" face |
| Steel floor register | 1-3/4" across the short side, 1-1/2" along the length | 4" x 10" opening, 5-3/4" x 11-1/2" face |
| Wood floor register | 1-1/2" across the short side, 1-7/16" along the length | 2" x 10" opening, 3-1/2" x 11-7/16" face |
| Steel toe kick register | 1-3/8" larger | 2" x 12" opening, 3-3/8" x 13-3/8" face |
| Steel baseboard supply register, multi-shutter damper | 1-1/4" larger | 10" x 6" opening, 11-1/4" x 7-1/4" face |
| Steel triangle baseboard grille or register | 1-3/4" in length, 5/8" in height | 12" x 6" opening, 13-3/4" x 6-5/8" face |
| Aluminum exterior gable vent | 2" larger | 6" x 6" opening, 8" x 8" face |
The face grows by the same amount on both sides of the opening, so a 1-3/4" gap means about 7/8" of overlap on each edge. That fixed gap is what lets you work backward from a face you already measured.
How to Convert a Face Measurement Into an Order Size
To convert a face measurement into an order size, subtract the face gap for that vent type from each dimension, then match the result to a standard opening. There are 3 common conversions:
- Subtract 1-3/4" from each side of a steel supply register face. A 13-3/4" x 7-3/4" face orders as a 12" x 6".
- Subtract 2-5/8" from each side of a steel return air filter grille face. An 18-5/8" x 22-5/8" face orders as a 16" x 20".
- Subtract 1-3/4" from the short side and 1-1/2" from the length of a steel floor register face. A 5-3/4" x 11-1/2" face orders as a 4" x 10".
Treat the result as a starting point when the old cover came from another brand, because brands and models can use different face gaps. A result that lands between two standard sizes, such as 11-1/2" x 6", means the gap does not match, so pull the cover and measure the opening itself. The subtraction also works only for vents ordered by the opening, and 4 vent types follow a different rule.
Vents That Are Not Ordered by the Duct Opening
The vents not ordered by the duct opening are T-bar lay-in diffusers, round ceiling diffusers, corner baseboard registers, and dryer vent covers. Each one sizes by a different part of the install.
The table below shows what to order by for each of these 4 vent types and how the face compares.
| Vent type | Order by | How the face compares |
|---|---|---|
| T-bar lay-in diffuser (drop ceiling, 2x2) | The 24" x 24" ceiling grid, plus the neck size | Panel measures 23-3/4" x 23-3/4", 1/4" smaller, so it drops into the grid |
| Round ceiling diffuser | Neck size, the diameter of the round duct | Face runs 3-15/16" larger than the neck, so an 8" neck has an 11-15/16" face |
| Corner baseboard register, steel or wood | The listed length, which is the overall length | An 18" register measures 18" long, 4-1/2" high, and 2-3/4" deep overall |
| Dryer vent cover | Neck size, the diameter of the dryer duct | Listed by neck: 4", 5", 6", or 8" |
The T-bar panel is the only vent here with a face smaller than its listed size. With the right rule for each vent, the last step is checking the size before you order.
Checking the Size Before You Order a Vent Cover
Checking the size before you order a vent cover comes down to 3 checks: match the listed opening to your measurement, compare the overall face to the space around the vent, and confirm the order-by rule on the product page. The questions below cover the cases that lead to wrong-size orders.
How do you read the size on a product page?
You read the size on a product page from 3 details: the nominal opening, the overall face dimensions, and the order-by rule. The nominal opening is the size you match to your measurement, and the overall face is the finished face. The order-by rule names what to measure, such as the duct opening, the neck size, or the space between baseboards.
What if the wall hole is bigger than the duct?
You should order by the duct, not the wall hole, when a drywall or floor cut runs larger than the duct behind it. A face with a 1-3/4" gap overlaps about 7/8" on each edge, so it hides a cut that runs a little wide. A cut wider than that overlap stays visible around the face, and a larger register body does not fit the smaller duct.
Do floor registers list the shorter side first?
Yes, floor registers list the shorter side first, and the two numbers are interchangeable because the register drops into a flat opening and turns to fit. A 4" x 10" floor opening takes a 4" x 10" register, and the guide on how to measure a floor register correctly covers depth and damper clearance. Wall and ceiling registers list width first, so a 10" x 6" and a 6" x 10" are two different products.
Does a new vent cover hide old screw holes and paint lines?
A new vent cover hides old screw holes and paint lines when its overall face is at least as large as the old face. Measure the old face before you remove it, then compare it with the Overall face dimensions on the product page. This is the one time the old face measurement matters, and it checks coverage, never the order size.
Should you measure every opening on a multi-room job?
You should measure every opening on a multi-room job, because one house can mix sizes such as 10" x 6" wall supplies, 4" x 10" floor registers, and a 20" x 20" return. Record each opening by room, surface, and supply or return, with width first for walls and ceilings and the shorter side first for floors.